Underground construction is basically unknown except for nuclear bunkers

Sunday, November 20th, 2022

Tunnels are a staple of both science fiction and popular journalism regarding human habitations on the Moon, Mars, or other rocky places, Casey Handmer notes:

They’re fun to write about and interesting to put on screen. I’ve lost count of the times I’ve seen beautifully illustrated Mars city maps featuring a hexagonal grid of domes connected by tunnels. On a visual level, it certainly ticks all the right boxes.

And yet, while I’ve wasted years of my life on real estate websites I’ve never seen a subterranean house on the market. They do exist, if you want a converted ICBM bunker or limestone cave, but they’re a definite rarity.


The simplest explanation is that digging holes, particularly really deep ones, is very energetically intensive and expensive. The cost of building a road tunnel works out to be about $100,000 per meter, or equivalent to a stack of Hamiltons of the same length! For comparison, $100,000 will buy materials and labor on a respectable manufactured home, or substantial renovations.

Indeed, on Earth, underground construction is basically unknown except for nuclear bunkers. These have two powerful reasons to accept the cost and inconvenience: unlimited sweet DoD money, and surviving really big explosions.

Why build underground in space? The usual explanation is to provide shielding against galactic cosmic rays, or micrometeorites.

It is true that tunnels deep underground are relatively safe from both, and also well thermally insulated. But as I discussed in the blog on space radiation, relatively little shielding is necessary even in areas that people spend a lot of time, such as sleeping areas. And even if that works out to be a meter or two of rock, it’s orders of magnitude less effort to drop sandbags on the roof of some structure constructed on the surface, than to dig a hole of the necessary size deep underground.

Micrometeorites are not a concern on Mars, which has a thin atmosphere, and can be well shielded on the Moon with a thin blanket of loose rubble.

If there’s a central point to my blogs on space architecture, it’s that our cities and houses on Mars will look and feel a lot more like regular houses on Earth, and for the same reasons. It may not be very exciting, but the most important consideration for design and construction, on Earth or in space, is expedience. Given the relative scarcity of human labor in space cities, structures will have to maximize usable area and minimize effort even more than on Earth. Instead of tunnels, think warehouses and aircraft hangars! At least they can have natural light.


  1. Roy in Nipomo says:

    Another reason other than price that prevents underground homes (at least in the US) is building codes. Most jurisdictions that I’m aware of demand windows in rooms used for living purposes (e.g. bedrooms, kitchen, etc) big enough for egress for safety reasons (e.g. fire exits, etc). Financial institutions probably go along. I’m sure that exceptions exist, but not enough to justify much research/innovation in lowering the cost.

    On the other hand, on Mars, the Moon, etc, one can always build a slightly below grade structure and then just pile surface material on top until the level of protection is achieved.

  2. Jim says:

    “Indeed, on Earth, underground construction is basically unknown except for nuclear bunkers.”

    It’s true that underground construction is basically unknown, but on the other hand, underground construction is basically unknown.

  3. Michael van der Riet says:

    On the other hand there are a lot of burrowing animals. Most plants prefer to dig under the surface. Microbes are fairly evenly split between those that like the outdoors and those that are happiest underground in a host. Perhaps we’re not looking at this the right way.

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